1. Field of the Invention
This invention relates to the determination of the surface condition of a body, and particularly to apparatus for determining the relative freedom from contamination of the surface of a metallic object. The present invention is useful not only in the testing of metals, including alloys, but may also be used with metalloids like germanium, carbon, silicon, boron or the like to determine their surface characteristics. For convenience in reference, the term "metallic" is used herein to include both metals and metalloids.
2. Description of the Prior Art
Contamination of surfaces of metallic parts is a serious problem in, for example, the manufacture of electronic components and electronic systems. Contamination on the surfaces of metallic contacts, for example, may cause considerable difficulty, such as intermittent continuity or even open circuits, and yet this contamination may not be detectable even when the contacts are viewed under a microscope. The bonding of parts to each other by the use of adhesives or by thermocompression techniques, for example, may be adequately strong initially but the bond may deteriorate under normal atmospheric conditions within a few weeks. This deterioration is apparently due to the penetration of moisture into the bonded joints along paths where the bond is weak. Weak bonding is often caused by the prior contamination of the surfaces.
Moisture also penetrates beneath poorly bonded conformal coatings on metallic objects. The moisture tends to concentrate in regions where there is ionic contamination due, for example, to residues of flux from soldering. Eventually the moisture may provide paths for electrical leakage currents and short circuits. In addition, the coatings may develop blisters. Similarly, penetration by moisture often causes metallic platings to blister.
The cost of implementing the controls necessary to achieve adequate surface cleanliness for those steps in the manufacture of, for example, electronic components has had a major impact on overall manufacturing costs in the electronics industry. The present invention represents the results of an effort to reduce such costs through the use of relatively inexpensive and non-destructive test instruments for monitoring surface cleanliness or surface condition. These instruments are suitable for use in on-line quality control procedures in manufacturing.
Surface potential difference (also known as contact potential difference) has been the object of considerable research. The principles involved are well documented in the literature.
Alexander Volta discovered in 1797 that two dissimilar metals placed in contact directly or by means of an intermediate conductor become charged with electricity of opposite polarity. A. Volta, Gehler's Worterbuch, IV, 616 (1801). Since that time, it has been established that the contact potential differences between metals of the same temperature are intrinsic properties of the metals although the measured potential difference is much altered from the intrinsic value by any intervening films on the metals. K. W. Bewig, C. O. Timmons, and W. A. Zisman, "Changes in Contact Potentials of Metals Caused by Adsorbed Monolayers," NRL Report 6200, Feb. 12, 1965. Theoretical research on the relation of contact potential differences to the electronic nature of metallic conductors and to the surface equilibrium of the electrons and ions in each metal lattice were reviewed in C. Herring and M. H. Nichols, Rev. Modern Phys., 21, 185 (1949). The use of contact potentials and the related work function of metals in the study of adsorption, and especially chemisorption, was reviewed in R. Suhrmann, "Changes in Conductivity on Adsorption of Gases on Metal Films," in "Chemisorption," W. E. Gardner, editor, London, Butterworths, p. 106, 1957 and in R. V. Culver and F. C. Tompkins, "Surface Potentials and Adsorption Process on Metals," in "Advances in Catalysts," Vol. XI, New York, Academic Press, p. 67, 1959. There is given in I. F. Patai and M. A. Pomerantz, J. Franklin Inst., 252, 239 (1951) a review of various techniques and apparatus for measuring the contact potential differences between metals. The apparatus discussed there includes various configurations of elements using the ionization method wherein a radioactive source is disposed on one of a pair of metallic electrodes or in the vicinity thereof to ionize the air between the electrodes and thereby "connect" them through a resistive path in the air. The potential difference between the electrodes so connected is measured by an electrometer.
3. Prior Art Statement
The most pertinent prior art discovered by Applicants relative to this invention is listed herewith.
(1) Muchnick, "Ascertaining Surface Condition," U.S. Pat. No. 3,009,100 issued Nov. 14, 1961.
(2) Ensanian, "Method for Measuring Surface Characteristics of Metals and Metalloids," U.S. Pat. No. 4,006,063 issued Feb. 1, 1977.
(3) Smith, Advan. Colloid Interface Sci., 3, 161 (1972)
(4) Guttenplan, "Evaluation of Surface Cleanliness by Surface Potentials"; unpublished paper presented orally at Second Seminar on Contamination: Its Effect, Detection and Control, Anaheim, Calif., Oct. 8-9, 1975.
(5) Hampel, C. A. ed., The Encyclopedia of Electrochemistry, pp. 236-239, Reinhold Publishing Corp., New York, 1964.
The patent to Muchnick discloses a method for determining the surface condition of a metallic body. The metallic body is incorporated into an electric circuit. An element or probe is moved close to the body but spaced apart therefrom to define an air gap. A unidirectional source voltage connected between the body and probe is varied to increase the voltage until the air in the gap is ionized and dielectric breakdown occurs. The voltage across the air gap just prior to breakdown is a measure of the condition of the surface of the metallic body. As an alternative to varying the voltage, the air gap can be narrowed until breakdown occurs at a suitable fixed voltage. The spacing at which breakdown occurs is then the measure of surface condition.
The patent to Ensanian discloses a method of mapping the surface condition of a metallic test specimen by measuring the mechanogalvanic potential between successive points on the surface of the specimen and a test probe or standard electrode. The probe is electrically connected to the specimen through a voltage measuring device. The circuit is completed through spaced apart droplets of an electrolyte sprinkled on the surface of the specimen. The electrolyte is a solution of a metallic salt. Alternatively, the circuit is completed through a rotatable non-metallic body of gelled electrolyte mounted on the test probe. One such probe or an array of such probes mounted to a common bracket may be rolled over the surface of the test specimen to map the variations in mechanogalvanic potential. The method of Ensanian is a destructive one in that the testing itself leaves a contaminating residue on the surface of the test specimen and thereby destroys the original character of the surface.
The article by Smith is a study of "Monomolecular Films on Mercury". The elaborate apparatus used for the study includes a reference electrode positioned about 0.5 cm above the surface of a body of mercury in a chamber. The electrode has a small amount of americium of mass number 241 sealed behind a stainless steel foil. The radioactive americium ionized helium cover gas which fills the chamber above the mercury. The surface potential of this arrangement is measured by a very high-impedance electrometer connected between the electrode and the mercury.
In the paper by Guttenplan, apparatus similar to that of Smith is disclosed with the difference that the reference electrode is mounted on a movable base having a three-point support. The height of the electrode relative to the base is adjustable. The paper discloses plans to incorporate such an electrode into a portable instrument having miniaturized electronic components therein for simply, inexpensively, and quickly providing qualitative measures of surface cleanliness. A plan to use such an instrument for controlling a production cleaning process is mentioned.
The encyclopedia entry shows a gas space formed by two plates with surfaces facing each other. The gas is ionized with a weak radioactive source, e.g., polonium 210. As long as there is a contact potential difference between the plates, a field exists between the surfaces and an ionic current flows through the space causing current detectable by a microammeter.